The present invention relates to systems and methods for locating faults in a power transmission system, more particularly, a power transmission system with multiple tapped loads.
Power transmission lines carry current from generating sources to electric power users. The power transmission lines are typically high voltage lines and are typically transformed down to a lower voltage at a power substation, before being distributed to individual electric power users (e.g., homes and business buildings). At many power substations, protective relays are included and perform the following functions in connection with the transmission system: (A) substation control and data acquisition and (B) protection. Data acquisition typically contains the functionality of (a) monitoring the system to ascertain whether it is in a normal or abnormal state; (b) metering, which involves measuring certain electrical quantities for operational control; and (c) alarming, which provides a warning of some impending problem. Protection typically involves fast tripping a circuit breaker in response to the detection of a short-circuit condition (a fault), typically within a few electrical cycles after a fault occurs
The detection of a fault in a protection function involves measuring critical system parameters and, when a fault occurs, quickly making a rough estimate of the fault location and of certain characteristics of the fault so that the faulted line can be isolated from the power grid as quick as possible. A fault occurs when a transmission line, typically due to external causes, diverts electrical current flow from its normal path along the transmission line.
The major types and causes of faults are insulation faults, caused by design defects, manufacturing defects, improper installation, and aging insulation; electrical faults, caused by lightning surges, switching surges, and dynamic overvoltages; mechanical faults, caused by wind, snow, ice, contamination, trees, and animals; and thermal faults, caused by overcurrent and overvoltage conditions.
A transmission line typically includes three phase lines, however, a transmission line may also contain one phase, or some other number of phases. With a three-phase transmission line, there are several types of possible faults. A single-phase fault is a fault from a single phase to ground (e.g. phase a to ground). A phase-to-phase fault is a fault from one phase to another phase (e.g., phase a to phase b). A phase-to-phase-to-ground fault is a fault that involves two phases and the ground (e.g., phase a and phase b to ground). A three-phase fault is a fault that involves all three phases and may or may not involve the ground (e.g., phase a, phase b, and phase c to ground).
In addition to protection functions, digital fault recorders or other processors may be included at a power substation or at a remote site for calculating fault locations. Fault location does not have to be as fast as protection function, which may be calculated after the fault has been handled by the protection function, but it should estimate the actual fault location more accurately than a protection function. Accurate fault location facilitates fast location and isolation of a damaged transmission line section, and quick restoration of service to utility customers after repair of the faulted line.
In addition to supplying power to an electrical user through a power substation with protective relaying, electrical utilities may also provide power to electrical users through a tap, referred to as a tap node. The tap is a connection point to a phase or phases of the power transmission system. There may be more than one tap node on a transmission system. The tap is connected to a tap lateral, which in turn is connected to and supplies power to a load, referred to as a tapped load. There may be more than one tapped load on a tap lateral. A tapped load typically does not have protective relaying, and therefore, does not usually have current and voltage data being measured/recorded.
Many fault location calculation systems exist for determining the location of a fault on a power transmission line. In these systems, voltage and current are measured at one or both ends of a segment of the transmission line. In some systems, the voltage and current measurements at both ends of a segment are synchronized. In a synchronized system, the voltage and current readings must have their time clocks synchronized. In some systems, data acquired before the fault condition is used in the calculation. Some prior fault location calculations are inaccurate for transmission lines with a tapped load, because they were designed for use on transmission lines without tapped loads. Some fault location calculations are only applicable to certain types of faults, thus a fault type must be selected before or during the calculation process, and the accuracy of these systems may be affected by the fault type selection.
The prior art does not address calculating fault locations on a power transmission line with multiple tapped loads, using synchronized or unsynchronized data from two ends (e.g., two protective relays providing current and voltage readings). In a power transmission line with multiple tapped loads, the calculations used previously yield less accurate estimations of the fault location. The fault location calculation on transmission lines with multiple tapped loads must resolve the main problems of the lack of measurement at the tap nodes, the fact that measurements at both ends of a tapped line may or may not be synchronized, and the fact that each tapped load normally is not a fixed load, but a varying load.
Therefore, a need exists for a system and method for calculating a fault location in a transmission line with multiple tapped loads using synchronized or unsynchronized measured data from two ends. The present invention satisfies this need.
The present invention is directed to systems and methods for calculating a fault location in a transmission line with multiple tapped loads using synchronized or unsynchronized measured data from two ends.
According to an aspect of the invention, a fault is located in a transmission line with a sending end, a receiving end, and multiple tapped loads connected to the transmission line at tap nodes. The fault location is determined by obtaining measured pre-fault and faulted current and voltage values at the sending end and at the receiving end of the transmission line. An equivalent tap node location is calculated using measured pre-fault and faulted current and voltage values at the sending end and at the receiving end of the transmission line.
The equivalent tap node divides the transmission line into a sending side and a receiving side. The sending end and the receiving end each include a measuring device. The phase angle difference due to unsynchronized measurement may be calculated using the measured pre-fault current and the measured pre-fault voltage values. The equivalent load impedance of the equivalent tapped load is calculated. A first fault location is calculated assuming that the fault is located on the sending side of the equivalent tap node. A second fault location is calculated assuming that the fault is located on the receiving side of the equivalent tap node. The fault location is selected from one of the first fault location and the second fault location, by selecting the fault location having a value within a predetermined range representing a full distance between two nodes.
According to another aspect of the invention, a fault location may be calculated for many types of faults.
According to a further aspect of the invention, a fault location may be calculated. for both single phase and three phase transmission lines.
According to another aspect of the invention, the measured data may be synchronized or unsynchronized.
These and other features of the present invention will be more fully set forth hereinafter.